The present invention generally relates to an image pick-up apparatus employing a solid-state image sensor. More particularly, the present invention is directed to a system for enlarging an image using electronic circuits, i.e., a system for zooming-in by means of electronic circuits.
While the semiconductor process technique typically utilized in a semiconductor memory has been improved more and more, the performance and yield of a solid-state image sensor (simply referred to as a "sensor" hereinafter) could be improved. As a result, an image pick-up tube employed in a video camera as the typical image pick-up apparatus has been substituted by this sensor. The major advantage of employing such a sensor in a video camera is to achieve a compact video camera. Under such a circumstance, various types of sensors have been recently developed in, for instance, "Quasi-field Integration CCD Color Camera" by M. Shimura et al, ITEJ Technical Report, Vol. 10, No. 52, pages 37 to 42, Feb. 1987. However, the zooming function of the conventional video camera, which is one of the necessary functions to the compact video camera, is realized by employing only the optical lens. Accordingly, there is a certain limitation on the magnification of the zooming lens in view of the conventional compact video camera.
On the other hand, since the memory capacity of the semiconductor memory is becoming larger and larger and also the cost thereof is becoming lower and lower, there is a tendency that the signal process in the video camera be performed by utilizing the memories.
Thus, if a picture is enlarged by utilizing the memories in the signal process, the entire optical lens system can be made compact. An image pick-up apparatus including such a picture enlarging system as represented in FIG. 1 may be conceive by the inventors of the present application In FIG. 1, reference numeral 1 indicates an optical lens; reference numeral 2 denotes a sensor; reference numeral 3 represents a sensor driving circuit; reference numeral 4 is an analog-to-digital converter (hereinafter abbreviated as an "A/D converter"); reference numeral 5 denotes a memory; reference numeral 6 represents a digital-to-analog converter (hereinafter abbreviated as a "D/A converter"); reference numeral 7 indicates a memory driving circuit; and, reference numeral 8 represents a switch.
In response to a driving pulse derived from the sensor driving circuit 3, a video signal "S.sub.N " as represented in FIG. 2, is obtained from the sensor 2. In FIG. 2, numerals "1" to "M" represent an i-th horizontal scanning period ("i" being 1, 2, - - - , M) during the vertical scanning period The video signal S.sub.N is analog-to-digital-converted by the A/D converter 4, and then stored into the memory 5. The signal stored in the memory 5 is slowly read out from the memory 5 by the memory driving circuit 7. Then, the readout signal is digital-to-analog-converted by the D/A converter 6 so as to obtain a zooming-in video signal "S.sub.z ".
A picture zooming-in operation will now be described in detail First, a zooming-in operation in the vertical direction will now be described. A 1/2 video signal in the vertical direction is stored in the memory 5 from an (M/4+1) horizontal scanning period to a (3/4M) horizontal scanning period. When the video signal is read out from the memory 5, the above-described video signal in the (M/4+1) horizontal scanning period is read out from the first horizontal scanning period to the second horizontal scanning period. Subsequently, the j-th ("j" being M/4+1, - - - , 3/4M) video signal is successively read out. As a result, the picture is enlarged twice in the vertical direction.
As to the zooming-in operation in the horizontal direction, the video signal is stored in the memory during a 1/2 time period of a center of the horizontal scanning period When the video signal is read out from the memory, the video signal is read out at a half frequency of the frequency at which the video signal is stored. Then, the picture is enlarged twice in the horizontal direction.
However, the following two items, are not considered in the above-described technique. That is:
(i) the deterioration in the vertical resolution, and PA1 (ii) the line misplacement.
A description will now be first made to the item (ii). When the video signal readout by the sensor 2 shown in FIG. 1 is carried out, it is necessary that in order to cancel an image lag, and also to correspond to an interlace scanning of a television monitor, the signals of photoelectric conversion elements arranged in two rows adjacent to each other in the vertical direction, the photoelectric conversion elements being arranged in a two dimensional form on a light receiving surface of the sensor, are simultaneously read under such a condition that either a combination of these adjacent two rows, or a pair of these rows are changed between the odd number field and even number field, as illustrated in FIG. 3. At this time, a luminance signal S.sub.k, .sub.k+1 is produced by adding the signal of k-th row to the Signal of (k+1)th row. As a result, when the image is enlarged, the luminance signals S.sub.k, .sub.k+1 ; S.sub.k, .sub.k+1 ; S.sub.k+2, k.sub.+3 ; S.sub.k+2, k.sub.+3 ; - - - are sequentially read out from the memory 5 in the odd number field, whereas the luminance signals S.sub.k+1, .sub.k+2 ; S.sub.k+1, .sub.k+2 ; S.sub.k+3, .sub.k+4 ; S.sub.k+3, .sub.k+4 ; - - - are successively read out from the memory 5 in the even number field. In FIG. 4, there is shown a case in which these luminance signals are reproduced by a television monitor. In FIG. 4, a dot line represents a scanning line in the even number field, whereas a solid line denotes a scanning line in the odd number field. There are shown the luminance signals S.sub.k, .sub.k+1 ; S.sub.k+1, .sub.k+2 ; - - - which are Successively displayed from the top portion of FIG. 4. At this time, as represented in FIG. 4, the luminance signals S.sub.k+1, .sub.k+2 ; S.sub.k, .sub.k+1, - - - are sequentially, mistakenly displayed in this order on the screen of the television monitor, instead of representing the luminance signals S.sub.k, .sub.k+1 ; S.sub.k+1, .sub.k+2 ; - - - on the screen in this order. This corresponds to the line misplacement, and the similar line misplacement occurs in the chrominance signal. As a consequence, the image quality of the horizontal edge portion (i.e., a rapid change in an image between a certain scanning line and a subsequent scanning line) of an object to be imaged is deteriorated.
Then, the deterioration of the vertical resolution as described in the item (i) will now be described. When the picture is enlarged, since the same luminance signals are displayed during two horizontal scanning periods, the vertical resolution is considerably deteriorated.